The explosive growth in the US prison population is well documented, but its causes are poorly understood. In this paper I exploit previously-unused data to define precisely where the growth is occurring. In short, the growth in prison populations has been driven almost entirely by increases in felony filings per arrest. All other possible sites of growth - arrests, admissions per filing, convictions per filings and admissions per conviction, and even (perhaps most surprisingly) time served per admission - have barely changed over the past four decades. But the growth in filings tracks that of admissions almost perfectly. This paper demonstrates the importance of felony filings and considers some of the possible explanations for their growth.